Amphibians
Greater Siren
Siren lacertina · also called greater siren, mud eel
The greater siren is a salamander that looks like it was assembled from an eel and a newt tadpole — a long, gilled body with a pair of stubby front legs, no back legs at all, and a lifelong refusal to grow up. It lives in the weedy shallows of southern swamps, eats plants as well as animals, and can wait out a three-year drought curled in dried mud.
Overview
Greater sirens live in heavily vegetated ponds, marshes, ditches, and slow streams of the US Atlantic and Gulf coastal plains. They are fully aquatic and nocturnal, hiding in mud and dense weed by day, and they are among the largest salamanders in North America at up to nearly a metre.
Front legs only, gills for life
The greater siren has a pair of small forelimbs behind its bushy external gills and no hind limbs or pelvis whatsoever. It moves by eel-like undulation. It is paedomorphic — it keeps its larval gills and never metamorphoses — but also has lungs and gulps air at the surface, and it makes clicks and faint yelps.
An omnivorous salamander
Almost all salamanders are strict carnivores. The greater siren eats invertebrates, small fish, and eggs, but also large amounts of algae and aquatic plants, which it appears to actually digest — one of the very few genuinely omnivorous salamanders.
Drought dormancy
When a wetland dries, the siren burrows down and secretes a hardening mucus cocoon, surviving at least two to three years in dried mud — the same trick as a lungfish — which lets it live in temporary wetlands that would kill a fish.
Status
The greater siren is Least Concern, common in suitable southeastern wetlands though difficult to survey, and long-lived — well past 25 years.
Greater Siren: common questions
What is a siren, and how is it different from an amphiuma?
Sirens and amphiumas are both large, eel-shaped, fully aquatic North American salamanders that live in the same kinds of muddy southeastern wetlands, and they are easy to confuse, but they belong to different families and differ in key ways. A siren keeps prominent bushy external gills for life and has only front legs (no hind limbs at all). An amphiuma has no external gills as an adult (just gill slits), and has four legs, though the legs are tiny and two-toed. Sirens also make sounds — clicks and faint yelps — and eat a lot of plant matter, which is very unusual for a salamander, while amphiumas are strict carnivores. The greater siren is the largest siren species.
Why does the greater siren keep its gills its whole life?
It is paedomorphic — it becomes sexually mature and reaches full adult size while retaining features of the larval stage, above all the feathery external gills, and it never undergoes the metamorphosis that turns a typical salamander larva into a land-living adult. Staying gilled and aquatic makes sense for an animal that lives permanently in warm, weedy, often low-oxygen water; the large gill surface helps extract oxygen, and the siren backs it up with lungs and by gulping air at the surface when the water is very poor. It has no need or ability to live on land.
How does it survive when its pond dries up?
It aestivates, like the lungfishes it superficially resembles. As the water disappears, the greater siren burrows into the mud, curls up, and secretes copious mucus from its skin, which dries into a parchment-like cocoon enclosing the whole body except the mouth. Inside, its metabolism drops drastically and it lives off stored fat and muscle. Sirens have been dug out alive from dried mud after two to three years, and the true limit is not known — this ability lets the species occupy temporary wetlands that would kill a fish or a normal salamander.
Does the greater siren really eat plants?
Yes, and this is one of the odd things about it. Nearly all salamanders are exclusively carnivorous. Greater sirens eat the expected invertebrates, small fish, and eggs, but analyses of their gut contents and their digestive physiology show that a substantial portion of their diet is filamentous algae and vascular plant material, and that they are actually able to assimilate nutrients from it rather than just passing it through. This makes the greater siren one of the very few salamanders that is a genuine omnivore, and it may help explain how the species reaches such high densities in some marshes.